\begin{tikzpicture}
    \pgfmathsetmacro{\R}{1.5}
    \pgfmathsetmacro{\r}{1.0}

    \tkzDefPoints{0/0/O_1, \R/0/A, \R+\r/0/O_2}
    \tkzDrawCircle[very thick](O_1,A)
    \tkzDrawCircle[very thick](O_2,A)
    \tkzDrawPoints(O_1, O_2)
    \tkzLabelPoints[right](O_1, O_2)
    \tkzLabelPoints[left](A)

    % 绘制外公切线 BC
    \tkzDefSimilitudeCenter[ext](O_1,A)(O_2,A)  \tkzGetPoint{J}
    \tkzDefLine[tangent from = J](O_1,A)  \tkzGetSecondPoint{B}
    \tkzDefLine[tangent from = J](O_2,A)  \tkzGetSecondPoint{C}
    \tkzDrawLine[very thick, add=0.5 and 0.4](B,C)
    \tkzLabelPoints[above](B,C)

    %
    \tkzDrawSegments[very thick](A,B  A,C)

    % 辅助线
    \tkzDefShiftPoint[A](0,1){a}
    \tkzInterLL(A,a)(B,C)  \tkzGetPoint{O}
    \tkzDrawLine[dashed, add=1.5 and 0.5](A,O)
    \tkzLabelPoints[right, yshift=.5em](O)
\end{tikzpicture}

